UNIVERSITY  OF  CALIFORNIA  PUBLICATIONS 

COLLEGE  OF  AGRICULTURE 
AGRICULTURAL  EXPERIMENT  STATION 

BERKELEY,  CALIFORNIA 


COTTONY  ROT  OF  LEMONS 
IN  CALIFORNIA 


BY 

CLAYTON  O.  SMITH 


BULLETIN  No.  265 

Berkeley,  Cal.,  January,  1916 


UNIVERSITY  OF  CALIFORNIA   PRESS 
BERKELEY 
1916 


Benjamin  Ide  Wheeler,  President  of  the  University. 
EXPERIMENT  STATION  STAFF 

HEADS    OF    DIVISIONS 

Thomas  Forsyth  Hunt,  Director. 
Edward  J.  Wickson,  Horticulture  (Emeritus). 

Herbert  J.  Webber,  Director  Citrus  Experiment  Station;  Plant  Breeding. 
Hubert  E.  Van  Norman,  Vice-Director;  Dairy  Management. 
William  A.  Setchell,  Botany. 
Myer  E.  Jaffa,  Nutrition. 

Eobert  H.  Loughridge,  Soil  Chemistry  and  Physics  (Emeritus). 
Charles  W.  Woodworth,  Entomology. 
Ralph  E.  Smith,  Plant  Pathology. 
J.  Eliot  Coit,  Citriculture. 
John  W.  Gilmore,  Agronomy. 
Charles  F.  Shaw,  Soil  Technology. 

John  W.  Gregg,  Landscape  Gardening  and  Floriculture. 
Frederic  T.  Bioletti,  Viticulture  and  Enology. 
Warren  T.  Clarke,  Agricultural  Extension. 
John  S.  Burd,  Agricultural  Chemistry. 
Charles  B.  Lipman,  Soil  Chemistry  and  Bacteriology. 
Clarence  M.  Haring,  Veterinary  Science  and  Bacteriology. 
Ernest  B.  Babcock,  Genetics. 
Gordon  H.  True,  Animal  Husbandry. 
James  T.  Barrett,  Plant  Pathology. 
Fritz  W.  Woll,  Animal  Nutrition. 
A.  V.  Stubenrauch,  Pomology. 
Walter  Mulford,  Forestry. 
W.  P.  Kelley,  Agricultural  Chemistry. 
H.  J.  Quayle,  Entomology. 
Elwood  Mead,  Rural  Institutions. 
J.  B.  Davidson,  Agricultural  Engineering. 
II.  S.  Reed,  Plant  Physiology. 
D.  T.  Mason,  Forestry. 

William  G.  Hummel,  Agricultural  Education. 
Leon  M.  Davis,  Dairy  Industry. 
John  E.  Dougherty,  Poultry  Husbandry. 
S.  S.  Rogers,  Olericulture. 
*Frank  Adams,  Experimental  Irrigation. 
David  N.  Morgan,  Assistant  to  the  Director. 
Mrs.  D.  L.  Bunnell,  Librarian. 

CITRUS    EXPERIMENT    STATION 
DIVISION    OF    PLANT   PATHOLOGY 

J.  T.  Barrett  H.  S.  Fawcett 

C.  O.  Smith 


*In  co-operation  Office  of  Public  Roads  and  Rural  Engineering,  U.  S.  D.  A. 


COTTONY  ROT  OF  LEMONS  IN  CALIFORNIA* 

By  CLAYTON  0.  SMITH 


The  decay  commonly  known  as  "cottony  rot"  or  "white  mold," 
has  been  associated  with  the  lemon  industry  probably  from  its  infancy, 
but  during  the  past  few  years  the  trouble  has  so  increased  in  import- 
ance as  to  demand  special  study.  The  disease  is  widely  distributed  in 
all  the  lemon-growing  sections  of  California,  but  is  not  known  to 
occur  on  Citrus  in  Florida.  The  fungus,  Sclerotinia  libertiana,  causing 
this  decay,  is  widely  distributed  on  other  hosts  and  probably  is  present 
in  other  lemon-growing  countries.  The  chief  loss  occurs  during  the 
winter  and  spring  months,  when  the  fruit  is  being  stored  and  cured 
in  tents  before  being  packed  for  shipment.  No  estimate  of  the  loss  in 
money  value  is  obtainable,  but  where  it  becomes  established  in  a  viru- 
lent form,  the  loss  must  be  considerable.  From  the  records  of  one  of 
the  lemon  houses  in  California  the  following  data  of  the  amount  of 
cottony  rot  from  January  to  July,  1911,  were  obtained.  During  this 
time  325  cars  were  in  storage.  Of  these,  97  cars  showed  amounts 
varying  from  1  to  23  boxes  each  affected  with  cottony  rot.  A  box 
having  this  trouble  means  a  large  percentage  of  decaying  fruit.  The 
decay  is  more  abundant  some  years  than  others  and  especially  so  when 
large  quantities  of  fruit  are  being  stored.  This  is  due  largely  to  the 
greater  difficulty  in  properly  inspecting  the  tents  and  in  detecting  the 
presence  of  the  fungus  before  it  has  caused  the  decay  of  a  large  num- 
ber of  fruits  by  contact  with  the  first  infected  ones. 

The  decay  on  the  fruit  is  chiefly  characterized  by  the  mass  of  white 
cotton-like  mycelial  growth  that  rapidly  spreads  over  the  infected 
lemons.  In  this  growth  are  to  be  found  black  seed-like  bodies  known 
as  sclerotia,  Figs.  1  and  5.  By  this  last  characteristic  the  cottony  rot 
decay  can  with  certainty  be  distinguished  from  other  lemon  rots.  In 
the  beginning  stages,  this  decay  closely  resembles  that  of  the  grey  mold 
or  Botrytis  rot,  and  at  this  time  the  two  are  often  confused  in  the 
packing  house.  In  this  early  stage  both  of  the  fungi  cause  a  softening 
and  discoloration  of  the  tissue.  The  affected  tissue,  while  somewhat 
softer  than  normal,  is  not  a  typical  soft  rot,  and  for  some  time  con- 
siderable firmness  of  the  tissue  may  remain.     In  the  advanced  stages, 


*  Paper   No.    22,    Citrus    Experiment    Station,    College    of    Agriculture,    Uni- 
versity of  California,  Riverside,  California. 

[237] 


238  UNIVERSITY    OF    CALIFORNIA — EXPERIMENT    STATION 

however,  the  tissue  becomes  more  completely  broken  down  and  watery. 
In  the  cottony  rot  as  a  usual  thing  the  infected  tissue  is  pale  lemon 
in  color,  while  in  that  of  the  grey  mold  it  is  darker,  being  almost 
chocolate.  When  the  aerial  growth  of  the  fungi  appears,  all  difficulty 
in  distinguishing  between  the  two  decays  disappears.  In  the  cottony 
rot  the  mycelial  growth  is  white  and  non-spore  producing,  but  contains 
hard  black  sclerotia.  In  the  grey  mold  (Botrytis)  there  are  numerous 
spores  that  can  readily  be  seen  in  mass.  The  whole  growth  and  spore 
masses  have  a  characteristic  greyish  color.  The  cottony  rot  has  no  dis- 
tinguishing odor  as  has  the  brown  rot  (Pythiacystis  citrophthora) ,  so 


Fig.  1. — Sclerotia  of  cottony  rot,  as  produced  under  cover  crops.    From  these 
the  peziza  or  spore  stage  grows. 


that  no  difficulty  should  be  had  in  deciding  between  the  decays  of 
brown  and  cottony  rot. 

The  twig  infection  of  citrus  by  the  cottony  rot  is  often  accompanied 
with  gumming.  The  infected  bark  is  of  a  cinerous  or  ash  color  and 
fibrous  in  its  advanced  stages.  The  characteristic  sclerotia,  Fig.  lb, 
are  often,  but  not  always,  present.  When  the  twig  is  at  length  sur- 
rounded by  the  fungus,  death  takes  place  suddenly,  the  leaves  become 
rolled-up  and  dead,  but  may  remain  attached  for  some  months.  This 
last  characteristic  is  the  most  noticeable  and  is  what  usually  attracts 
the  grower's  attention  to  this  disease  in  the  orchard. 


COTTONY    ROT    OF   LEMONS   IN    CALIFORNIA  239 


NATURAL  INFECTION 

Picking-Box  Infection. — It  is  only  occasionally  that  the  conditions 
are  favorable  for  cottony  rot  to  develop  in  the  picking  boxes,  as  the 
fruit  remains  in  these  only  a  short  time  before  being  washed.  Several 
instances,  however,  have  come  to  our  notice  in  which,  under  favorable 
conditions,  a  large  amount  of  this  decay  (possibly  as  much  as  75  per 
cent)  did  develop.  The  fruit  was  picked  and  left  out  in  a  rain  of 
several  days'  duration  without  protection.  Under  these  conditions  not 
only  a  great  amount  of  decay  was  to  be  detected  when  the  fruit  was 


Fig.  2. — Peziza  or  spore  stage  showing  varying  lengths  of  stem  from  sclerotia 
to  the  funnel-shaped  disk,  apothecia,  of  cottony  rot  fungus.  The  length  of  stem 
varies  according  to  the  depth  below  the  surface  of  the  sclerotium. 


washed,  but  this  continued  to  appear  during  the  process  of  curing. 
In  a  few  localities  the  fruit  is  stored  in  the  picking  boxes  and  no  wash- 
ing of  the  fruit  is  considered  necessary.  In  these  cases  the  picking  box 
becomes  also  the  storage  box. 

Packing -House  Infection. — The  infection  of  fruit  in  the  packing 
house  is  the  chief  injury  done  by  the  cottony  rot  fungus.  The  fruit 
is  left  in  the  curing  tents  or  tight  rooms  from  one  to  two  months  after 
being  washed,  when  the  final  sorting,  grading  and  packing  of  the  fruit 
takes  plaee.  During  this  time,  especially  on  fruit  unusually  weak,  the 
cottony  rot  has  a  favorable  chance  to  develop.  The  decay  is  found 
almost  entirely  during  the  spring  and  winter  months,   January  to 


240 


UNIVERSITY   OF    CALIFORNIA — EXPERIMENT    STATION 


May,  rarely  to  June.  After  this  time,  the  decay  almost  entirely  dis- 
appears. The  lemons  during  curing  are  kept  under  rather  moist  con- 
ditions to  prevent  wilting,  and  in  the  stacked  boxes  the  conditions  are 
ideal  for  Sclerotinia  to  develop.  The  rot  when  once  started,  becomes 
virulent  and  a  large  mass  of  fruit,  even  green  fruit,  may  become  in- 
volved (Figs.  4  and  5).  The  fungus  secretes  a  substance,  perhaps  an 
enzyme,  that  has  the  power  of  softening  the  tissue  of  a  healthy  lemon 
and  so  prepares  the  way  for  the  mycelium  to  enter.  When  the  disease 
once  becomes  started  in  a  half  box  of  lemons,  almost  every  lemon  soon 
becomes  infected  and  the  decay  often  spreads  upward  and  downward, 
extending  through  the  entire  stack  of  fruit.  Workers  in  the  packing 
house  have  the  idea  that  the  drip  from  the  rotting  of  the  fruit  into  the 

box  containing  sound  fruit  beneath, 
has  much  to  do  with  the  spread  of 
the  fungus  through  the  stack  of 
fruit.  This  has  never  been  defi- 
nitely proved,  but  without  question 
the  mycelium  does  grow  through 
the  open  spaces  in  the  bottom  of 
the  boxes  to  sound  fruit  below. 

Care  should  be  exercised  in  the 
handling  of  contact  fruit ;  that  is, 
fruit  that  has  been  touching  cot- 
tony rot  decay.  This  will  be  con- 
sidered in  detail  later  under  "con- 
trol methods."  In  most  respects 
the  treatment  is  similar  to  that 
recommended  for  brown  rot,  and 
with  this  most  packing  house  work- 
ers are  familiar. 


Fig.  3. — Section  showing  how  the 
spores  are  produced  in  long  sacks  or 
asci  that  form  the  spore-bearing  sur- 
face of  the  f  angus. 


Seed-Bed  Infection. — The  cottony  rot  is  not  regarded  as  a  serious 
disease  of  citrus  seed-bed  stock.  It  has,  however,  been  found  occasion- 
ally attacking  sour  and  sweet  orange  seedlings.  The  shaded  moist 
condition  of  the  seed  bed  covered  with  lath  or  burlap,  gives  ideal  con- 
ditions for  the  fungus  to  thrive.  The  disease  usually  shows  in  small 
patches  of  six  to  a  dozen  trees.  It  is  thought  to  gain  entrance  into 
sound  tissue  by  starting  as  a  saprophyte  on  weakened  or  fallen  leaves 
that  are  in  close  contact  with  healthy  leaves  or  stems.  Trees  in  the 
seed  bed  often  grow  very  close  together.  Seedlings  iy2  to  3  feet  high 
have  been  found  to  be  attacked,  although  there  is  no  reason  why  those 
smaller  or  larger  might  not  be  infected  as  easily. 


COTTONY    ROT    OF    LEMONS   IN    CALIFORNIA 


241 


Nursery  Infection. — The  disease  only  rarely  does  any  marked  in- 
jury on  citrus  nursery  trees.  It  usually  attacks  trees  scattered  here 
and  there  throughout  the  nursery  and  shows  no  evidence  of  spreading 
to  adjacent  healthy  ones  (Fig.  6).  The  diseased  trees  show  the  same 
characteristics  as  do  the  infected  twigs  of  orchard  trees  and  should 
be  given  the  same  treatment. 

Orchard  Injection. — Among  citrus  trees  the  lemon  is  more  often 
attacked  than  the  orange.  The  disease  appears  to  be  localized  on  a 
branch,  the  infection  often  occurring  some  distance  from  the  end  of 
the  twigs.  Smaller  twigs  are  more  often  infected,  although  vigorous 
growing  sprouts  are  also  attacked.     The  twigs  may  become  infected 


Fig.  4. — Cottony  rot  affecting  lemons  by  contact  in  a  packinghouse  box. 


through  the  blossoms  or  through  some  injury.  The  disease  is  often 
worse  following  a  cool  winter  when  the  tissue  is  weakened  or  injured 
by  frost. 

Definite  infected  areas  developed  on  the  twig.  These  increase  in 
size,  soon  entirely  girdling  the  twig  and  causing  the  sudden  death  of 
the  part  beyond  the  place  of  infection.  The  disease  does  not  extend 
either  up  or  down  the  infected  branch  to  any  great  distance.  The 
infected  area  at  first  shows  a  softening  of  the  bark  tissue  with  a  con- 
siderable amount  of  gumming  (Fig.  7a).  The  bark  of  the  diseased 
area  is  often  of  a  lighter  color  and  at  length  shows  a  fibrous  condition 
(Fig.  7c).  With  the  appearance  of  the  gumming  of  the  twig  the 
spread  of  the  infection  appears  to  be  checked.  On  an  infected  twig 
the  leaves  remain  for  a  long  time  in  a  rolled-up  dead  condition  (Fig. 


242  UNIVERSITY   OF    CALIFORNIA EXPERIMENT    STATION 

8) .  Small  twigs  are  more  frequently  attacked,  although  vigorous  grow- 
ing sprouts  may  also  be  infected.  The  infection  may  sometimes  start 
at  the  end  of  the  twig  and  spread  for  a  short  distance  back  into  the 
more  vigorous  wood.  In  these  cases  the  spread  of  the  disease  can  often 
be  shown  to  follow  injury  from  frost  of  the  preceding  winter,  although 
other  conditions,  as  unusually  long  periods  of  warm,  rainy  weather 
in  winter  or  spring,  also  favor  twig  infection. 

The  cottony  rot  fungus  has  been  found  occasionally  attacking  the 
twigs  and  trunk  of  small  lemon  trees  that  have  been  wrapped  with 
corn  stalks  for  winter  protection.  This  fungus  has  killed  young  trees 
that  have  been  planted  in  the  orchard  one  year  from  the  nursery.  A 
case  was  observed  by  Professor  H.  S.  Fawcett  in  which  the  fungus 
had  secured  a  good  foot-hold  in  the  trunk  and  roots  of  a  large  lemon 
tree,  which  had  been  weakened  by  attacks  of  gummosis.  This  tree  had 
been  wrapped  with  manure  and  burlap  to  keep  the  tissue  from  drying 
out  so  as  better  to  favor  healing,  thus  affording  ideal  conditions  for 
fungous  growth,  and  the  formation  of  characteristic  sclerotia. 

Under  favorable  conditions  the  blossoms  may  become  diseased,  the 
infection  from  here  spreading  into  the  fruiting  twigs.  It  is  believed 
that  blossom  infection  most  often  starts  on  the  older  petals  which, 
instead  of  falling,  may  remain  attached  in  a  more  or  less  decadent 
condition  susceptible  to  the  attack  of  this  fungus  ( Fig.  9 ) .  The  blos- 
soms of  lemons  often  grow  in  large  clusters  and  a  considerable  mass 
of  collapsed  petals  are  present  where  spores  can  germinate  and  begin 
growing  as  a  saprophyte,  at  length  acquiring  a  parasitic  nature  capable 
of  infecting  healthy  tissue.  This  same  fungus  at  times  is  a  parasite  on 
the  blossoms  of  apricots,1  attacking  them  while  still  enclosed  by  the 
calyx  or  outer  cup  of  the  blossom. 

The  leaves  of  lemons  are  sometimes  attacked  while  they  are  at- 
tached to  the  tree.  There  is  not,  however,  any  marked  defoliation, 
probably  only  the  older  and  weaker  leaves  being  susceptible. 

Cover-Crop  Infection. — There  is  no  question  but  what  the  cottony 
rot  decay  has  increased  with  the  increased  growing  of  cover  crops. 
The  same  fungus  not  only  occurs  as  a  rot  of  the  lemon,  but  is  also 
capable  of  becoming  a  parasite  on  certain  cover  crops,  vetch  being  by 
far  the  most  common  host.  "While  the  fungus  is  common  on  vetch,  the 
injury  it  does  to  the  crop  as  a  whole  is  negligible.  The  dense  growth 
of  vetch  affords  a  suitable  place  for  the  fungus  to  grow  as  a  saprophyte 
or  weak  parasite  on  the  weaker  shaded  portion  of  the  vines.  (Fig.  10) 
Under  the  vetch  cover  crop  the  sclerotia  are  produced  in  large  numbers 


i  Bulletin  No.  218,  California  Experiment  Station,  p.  1097. 


COTTONY    ROT    OF    LEMONS   IN    CALIFORNIA 


243 


Fig.  5. — Cottony  rot  affecting  lemons  by  contact  in  a  packing-house  tray. 


244  UNIVERSITY   OF    CALIFORNIA — EXPERIMENT    STATION 

and  can  be  found  in  infested  orchards  with  little  searching.  The  more 
upright  cover  crops,  like  Melilotus  indica,  would  probably  be  less  favor- 
able for  the  development  of  the  fungus,  although  this  phase  of  the 
subject  has  not  been  thoroughly  investigated.  The  spore  stage  has 
been  found  in  orchards  having  a  straw  mulch,  but  never  in  so  large 
numbers  as  with  vetch  (Fig.  11).  There  is  no  question  of  a  definite 
relationship  existing  between  this  fungus  and  some  of  the  common 
cover  crops  in  use,  but  this  apparently  should  not  in  any  way  dis- 
courage their  use.  The  control  of  the  fungus  must  be  sought  for  in 
other  ways  than  through  discontinuing  the  use  of  green-manure  crops. 

CAUSE  OF  DISEASE 

The  decay,  as  already  suggested,  is  due  to  a  fungus  belonging  to 
the  genus  Sclerotinia.  From  the  preliminary  study  thus  far  made,  it 
is  believed  that  the  fungus  is  Sclerotinia  libertiana,  although  another 
species  of  this  genus,  probably  Sclerotinia  trifoliorum,  is  able  to  de- 
velop on  lemons  from  artificial  inoculations  and  possibly  may  cause 
some  of  the  infection. 

Relation  to  other  Hosts. — This  fungus  is  not  restricted  to  citrus 
for  a  host,  but  is  an  important  trouble,  attacking  various  other  plants, 
especially  seedlings,  certain  of  the  vegetables,  green-manure  plants, 
and  weeds.  The  following  are  some  of  the  hosts  known  to  be  affected 
by  the  lemon  Sclerotinia,  or  one  that  closely  resembles  it  pathogenic- 
ally  and  morphologically :  Lemon  fruit,  twig  and  blossoms  of  lemon, 
orange  twigs,  apricot  blossoms,  rarely  avocado  twigs  and  blossoms, 
lettuce,  tomato,  alfalfa,  ginseng,  celery,  bean,  cucumber,  vetch,  nettle 
(Urtica  urens)  and  prickly  lettuce  (Lactuca  scariola),  two  common 
weeds  in  citrus  orchards,  and  occasionally  the  leaves  and  weaker  stem; 
of  barley  and  oats.  As  a  saprophyte,  the  fungus  could  attack,  under 
favorable  conditions,  many  sorts  of  dead  organic  matter  and  decaying 
plants. 

From  spore  measurements  and  other  microscopic  characteristics 
of  the  cottony  rot  fungus,  it  has  always  been  regarded  as  the  species, 
libertiana.  To  determine  more  fully  this  point,  cultures  of  Sclerotinia 
libertiana,  were  requested  from  several  of  the  Experiment  Stations. 
Puncture  inoculations  from  these  were  made  in  the  twigs  of  Eureka 
lemon,  as  well  as  on  the  fruit.  The  results  of  these  are  summarized 
under  artificial  inoculations,  Table  I,  p.  252,  and  in  almost  every  case 
gave  definite,  positive  results.  There  remains  little  doubt  of  the  cot- 
tony rot  fungus  being  identical  with  Sclerotinia  libertiana. 

The  following  are  the  different  hosts  and  localities  from  which 
these    cultures    were    secured:     (1)    Several    different    cultures    from 


COTTONY    ROT    OF   LEMONS   IN    CALIFORNIA 


245 


lemon  fruits,  California;  (2)  six  or  more  different  strains  from  lemon 
twigs,  California;  (3)  alfalfa,  California  and  Oregon  Experiment 
Stations;  (4)  cucumber,  California  and  Oregon  Experiment  Stations; 
(5)  lettuce,  California  and  Ex- 
periment Stations  of  North  Caro- 
lina, Maine  and  Wisconsin;  (6) 
eggplant,  California;  (7)  tomato 
in  greenhouse,  California;  (8) 
celery,  Calif ornia ;  (9)  bean  stem, 
Maine  Experiment  Station ;  (10) 
snapdragon,  Maine  Experiment 
Station;  (11)  ginseng,  from  Cor- 
nell University;  (12)  avocado, 
California.  This  last  fungus 
was  isolated  by  Professor  H.  S. 
Fawcett  from  twigs  of  avocado 
which  had  been  killed  from  infec- 
tion that  appeared  to  have  start- 
ed in  the  blossoms  and  spread 
into  the  more  woody  twigs. 

Life  History  of  Fungus. — 
The  life  history  of  the  fungus 
consists  of  three  stages,  differing 
in  appearance  and  functions : 
(1)  The  vegetative  stage  is  the 
white  cotton-like  growth  so  com- 
mon in  the  packing  houses.  This 
growth  has  no  spores,  but  can 
spread  rapidly  and  has  the  power 
to  infect  sound  fruit  simply  by 
coming  in  contact  with  and 
spreading  over  it.  (2)  In  this 
white  growth,  numerous  black 
bodies  known  as  sclerotia  are  de- 
veloped (Fig.  1).  These  are 
nothing  more  than  a  very  solid 
dense  mass  of  mycelium,  so  form- 
ed that  they  will  resist  long 
periods  of  hot  dry  weather  or  the  cold  freezing  conditions  of  the 
eastern  states.  After  these  bodies  have  gone  through  a  period  of 
rest  and  the  favorable  rainy,  cool  weather  of  the  autumn  and  winter 


Fij 


. — Eureka  lemon  tree  in  nursery 
affected  with  cottony  rot. 


246 


UNIVERSITY   OF    CALIFORNIA — EXPERIMENT    STATION 


has  come,  they  revive  and  become  active.  The  sclerotia  may  then  send 
out  a  fresh  growth  of  mycelium,  the  vegetative  stage,  which  could 
easily  infect  dead  organic  matter  or  the  weaker  plant  growth  of  weeds 


Pig.  7. — Cottony  rot  affecting  the  twigs  of  a  Eureka  lemon  tree,  (a)  Masses 
of  gum  produced  by  the  fungus,  (b)  Sclerotia  of  the  fungus  distinguished  from 
the  gum  globules  by  more  spherical  shape,     (c)  Fibrous  condition  of  infected  bark. 

or  cover  crops.  Here  after  a  time  new  sclerotia  are  again  formed. 
(3)  From  the  sclerotia  in  the  soil  the  spore  stage  is  formed  under 
the  cover  crops  in  the  lemon  orchards.  This  last  is  accomplished  by 
sending  out  a  growth  which  eventually  develops  into  a  small  funnel- 


COTTONY    ROT   OF   LEMONS   IN    CALIFORNIA 


247 


shaped,  cup-like  disk,  technically  called  an  apothecium  (Fig.  2).  In 
the  flesh  of  the  apothecia  are  produced  the  spores.  These  are  formed 
in  rather  long  cylindrical  sacks  or  asci  (Fig.  3).  Hence  the  spores  are 
called  ascospores.  Both  the  spores  and  asci  are  microscopic.  The 
spores  are  easily  scattered  by  the  wind.  The  fungus  also  has  the 
power  to  force  out  or  discharge  these  spores,  and  they  can  often  be 
observed  leaving  the  apothecium  as  small  clouds  of  dust.     Some  of 


Fig.  8. — Artificial  inoculations  of  twigs  of  Navel  orange.  Note  the  rolling  of 
leaves  which  have  become  dead  and  bleached  to  almost  a  white  color  and  yet 
still  remain  attached  to  branch. 


these  spores  are  carried  by  the  wind  to  the  fruit  and  twigs,  of  the 
lemon  and  cover  crops,  where  under  favorable  conditions  they  ger- 
minate and  produce  the  vegetative  stage. 

The  apothecial  stage  in  California  begins  to  appear  about  the  first 
of  October  and  continues  to  be  formed  during  the  winter  and  spring. 
After  the  cover  crops  of  the  orchards  are  plowed  under  and  cultiva- 
tion begins,  few  apothecia  (spore  stage)  are  thought  to  be  produced, 
for  this  stage  of  the  fungus  requires  moist,  shaded  conditions,  such  as 
are  best  found  under  cover  crops  during  the  rainy  season. 


248  UNIVERSITY   OP    CALIFORNIA — EXPERIMENT    STATION 

ARTIFICIAL  INOCULATION  EXPERIMENTS 

On  Fruit. — The  disease  can  be  readily  produced  on  the  ripe  or  green 
lemons  when  left  in  a  moist  chamber:  (1)  By  placing  the  mycelium 
or  sclerotia  in  the  tissue  through  a  puncture  or  injury;  (2)  by  plac- 
ing mycelium  on  the  uninjured  surface  of  the  fruit — this  is  especially 
virulent  if  some  other  dead  organic  matter  is  also  present;  (3)  by 
placing  a  piece  of  an  apothecium  in  the  tissue  through  a  puncture  or 
injury ;  (4)  by  atomizing  spores  on  the  surface,  in  which  case  infection 
occurs  only  at  the  button  and  blossom  ends  and  at  points  in  contact 
with  each  other  or  with  the  moist  chamber;  (5)  by  inoculating  the 
stem  and  blossom  ends  with  mycelium  of  the  fungus,  infection  devel- 
oped in  two  weeks'  time. 

Apothecia  placed  in  drops  of  sterilized  distilled  water  on  the  surface 
of  lemons  gave  negative  results.  Many  spores  must  have  been  lib- 
erated in  the  water,  but  the  germ  tubes  were  not  able  to  infect  the  un- 
injured rind.  This  observation  agrees  with  those  of  former  investi- 
gations that  the  mycelium  of  this  fungus  needs  to  grow  for  a  time 
as  a  saprophyte  before  it  becomes  actively  parasitic.  The  reason  for 
this  requirement  is  perhaps  explained  by  the  fact  that  the  mycelium 
secretes  an  enzyme  or  substance  that  apparently  plays  a  very  import- 
ant part  in  the  softening  and  breaking-down  of  the  tissue,  thus  pre- 
paring the  way  for  its  invasion  with  the  fungous  hyphae.  Spores 
placed  in  drops  of  water  or  in  drops  of  prune  juice  give  negative 
results  on  uninjured  tissue. 

Typical  experiments  in  inoculating  lemons  with  cottony  rot.  From 
these  and  numerous  other  experiments  conclusions  as  given  under 
artificial  inoculations  were  largely  derived : 

1.  Experiments  in  atomizing  lemons  in  a  moist  chamber  with  spores  of  the 
fungus.  This  is  only  one  of  many  made  during  the  investigation.  The  others, 
however,  show  practically  the  same  results. 

(a)  In  this  experiment  an  attempt  was  made  to  imitate  as  nearly  as  pos- 
sible natural  conditions.  On  February  27th,  1911,  five  nearly  ripe  lemons  were 
secured  from  an  orchard  that  had  always  been  free  from  this  decay.  The 
fruit  was  picked  during  a  rainy  period  beginning  on  February  25th  and  con- 
tinuing more  or  less  until  March  2d.  The  fruit  wTas  first  sterilized  for  five 
minutes  in  4  per  cent  formalin*  and  then  rinsed  in  tap  water  and  left  out 
in  the  rain  for  a  short  time  before  being  atomized  with  spores.  The  fruit 
was  then  placed  in  a  quart  fruit  jar  for  a  moist  chamber,  and  left  out  in  the 
rain  where  the  temperature  was  somewhat  cooler  than  that  in  the  laboratory. 
When  the  rain  ceased  the  jar  was  covered  with  a  large  plant  pot  to  exclude 
the  light.     In  five  days  the  fruit  was  examined  and  found   to  be  infected   at 


*  An  aqueous  solution  of  40  per  cent  formaldehyde  is  a  commercial  product,  and 
is  called  ''Formalin". 


COTTONY    ROT   OP    LEMONS   IN    CALIFORNIA  249 

all  points  where  it  was  in  contact.  Four  of  the  five  showed  infection  at  the 
stem  end.  At  this  time  there  was  no  evidence  of  aerial  mycelial  growth,  but 
simply  a  small  amount  of  tissue  infected  and  softened  at  all  points  where  the 
fruits  were  touching  each  other  and  at  the  stem  end.  Characteristic  mycelium 
and  sclerotia  developed  later. 

(b)  Secured  from  lemon  orchard  fruit  showing  the  small  beginning  spot 
of  brown  rot  (Pythiaeystis  citrophthora) .  The  fruit  at  this  time  is  only  slightly 
pitted  or  depressed  at  point  of  infection.  Sterilized  the  brown  rot  fruit  in 
4  per  cent  formalin  for  five  minutes  and  rinsed  same  in  tap  water.  No  injury 
by  germicide  was  observed  on  these  lemons.  On  February  25,  1911,  this  fruit 
was  atomized  with  spores  of  cottony  rot.  On  March  8th,  the  brown  rot  had 
spread  over  about  one-third  of  the  surface  of  the  lemons.  The  cottony  rot  was 
also  developing  over  the  brown  rot  areas  and  had  invaded  some  of  the  tissue 
not  yet  infected  by  the  brown  rot  fungus.  On  March  14th,  the  cottony  rot 
had  completely  overgrown  and  covered  the  entire  fruit.  The  cottony  rot 
seems  to  attack  readily  tissue  in  which  the  brown  rot  has  become  established, 
in  brown  rot  fruit  kept  as  a  check  without  being  inoculated  with  cottony  rot 
spores,  no  cottony  rot  appeared. 

2.  Experiments  in  soaking  lemons  in  water  inoculated  with  spores  of 
cottony  rot.  Experiments  were  made  at  several  different  times  following  in 
a  somewhat  limited  way  methods  similar  to  those  used  in  inoculating  lemons 
with  the  brown  rot. 

A  receptacle  of  sufficient  size  to  contain  twelve  lemons  was  filled  with 
sterile  water  inoculated  with  macerated  apothecia  of  cottony  rot.  On  Decem- 
ber 28,  1910,  soaked  lemons  in  this  solution  for  36  hours,  and  at  the  end  of 
two  weeks'  time  there  was  no  evidence  of  decay.  February  3,  1911,  the  experi- 
ment was  repeated  with  thirteen  green  lemons  soaked  in  tap  water  mixed  with 
six  large  apothecia  secured  from  under  a  vetch  cover  crop.  The  apothecia  were 
first  macerated  in  small  pieces,  and  the  fruit  was  left  for  24  hours.  On  Marcli 
29,  1911,  the  fruit  showred  no  evidence  of  decay. 

3.  Experiments  in  inoculating  the  fruit  in  moist  chamber  through  punctures 
and  other  mechanical  injuries.  This  method  rarely  fails  of  giving  positive 
results. 

February  15,  1911,  inoculated  three  lemons  in  moist  chamber  with  pieces 
of  the  apothecia,  first  making  a  slit  in  the  rind  of  fruit  on  side  and  at  stem 
end.  These  apothecia  came  from  two  different  sources:  (1)  from  the  orchard 
under  a  vetch  cover  crop;  (2)  from  cultures  isolated  from  a  diseased  orange 
branch.  February  25,  1911,  all  the  inoculated  lemons  showed  infection  at  the 
points  of  inoculation. 

4.  Experiment  in  inoculating  fruit  by  placing  mycelium  on  the  uninjured 
surface. 

(a)  December  28,  1910,  inoculated  stem  or  button  end  of  three  lemons  with 
mycelium  taken  from  moist  chamber  where  the  cottony  rot  fungus  was  grow- 
ing on  a  lemon.  January  15,  1911,  slight  indication  of  softening  of  tissue 
around  stem.  February  16,  1911,  area  about  stem  had  softened  in  every  case 
and  later  developed  typical  cottony  rot. 

(b)  December  22,  1910,  placed  mycelium  from  a  cottony  rot  lemon  on  the 
uninjured  surface  of  five  lemons.  January  3,  1911,  showed  diseased,  softened 
areas  about  an  eighth  of  an  inch  in  diameter  in  each  place  where  the  surface 
was  inoculated.     Typical  cases  of  cottony  rot  soon  developed. 


250 


UNIVERSITY   OP    CALIFORNIA — EXPERIMENT    STATION 


■f: 
P 
O 

3 

< 
> 

o 

w 
o 

S 

QJ  CD  iD  <X>  CD  CD        CD 


<u 

cd 

cd 

Q3 

cd 

© 

cd 

cd 

CD 

CD 

CD 

t> 

> 

> 

> 

> 

> 

> 

t> 

> 

> 

l> 

"fl.    -^ 

H)J 

H-3 

+= 

'+= 

+3 

"-2 

55     <S 

c3 

oi 

e3 

e& 

e3 

OS 

cS 

cS 

oS 

pi 

o>    bo 

bo 

bfl 

be 

to 

bC 

bo 

bC 

bo 

bfl 

bfl 

Ph   ® 

<u 

<a 

cd 

CD 

© 

CD 

CD 

CD 

CD 

CD 

£ 

fc 

£ 

fc 

£ 

£ 

£ 

£ 

£ 

£ 

£ 

05      g 

s 

s 

s 

S 

s 

0 

s 

s 

g 

g 

p 

3 

1= 

3 

fl 

fl 

3 

^ 

2 

5 

£E 

Ph 

Ph 

Oh 

Ph 

Ph 

Ph 

s 

Ph 

Ph 

Pm 

f-1  »  c«  os  a  a  d  os     os  ctrQ^^ 


= 


£    bfl  bfl  be  be  be  bo     be  bo  3  3  3 

Ph  pH     CD  CD  CD  CD  CD  CDCD  CD  O  O  O 


EH 

Q      CH  "A       £  H3  r-J  i— ; 

^g-d    jd    xi    xi     „    .22  rS  _ca    xi     *     «     eg 
^  2 
g^^Pn     Pm     Ph     Ph     O     H     Ph     Ph     <1     < 

ajEHcpcDaja)  cdcd 

br      O  >>  i>  L>  t»  CD  CD  £>  !>  CD  CD 


r— '        PS  CJ  CD  W 

*>   oS  oS  oS  oS  £  b0>£  OS  ?  ©  p 

feCD  CD  CD  CDr5  C       ^         CD  CD  >-*  ►"*  r*" 


» 


OS 


y       W        'S    H^>  H^>  -+3  H-3  .Jh  .P  -U  -(J  .!_,  .Jh  .Jh 

E-nJ^oS  oS  a  oS  -u  -+3  oS  oS  -+J  -t-3  -*j 

^     £       3     ??  5?  SP  5?  "S  '3  W)  »  '5  "S  *S 

J    (B     K    »  ©  ©  ©  o  o  ©  ^  o  o  o 

gW££££PHPH££PHPHPH 

I  g  «1    1    T    1     a    a    1    1     a    a    a 


H    Q 


a  a         t1         t"     ■     h  fl  fl 


..     ^    si     >  a  a  g  g^os  cs  £.£,«*  os  os 

3G§h<i1  <  <1  <i  o  o  <  <  o  o  o 

<3        Eh     D 

t^        J~j     CQ  QjcDcu<u<uciJCDOCDCDCD 

EH         ^    .t  .^  .^  .fe  -M  -M  -M  -M  .^  .^  .£ 

H^  3+1  +=  +=  +=  +=>  +=  [-^  -+f  "+=  '-2  "-+3 

^  /T^  S  'w  *M  'w  ^  "M  "oq  '5°  M  "m  *M  'm 

"CLi^O  o  o  o  o  o  o  o  o  o  o 

w^^Ph  Ph  Ph  Ph  Ph  Ph  Ph  Ph  Ph  Ph  Ph 

izkS^c  a  s  c  pi  a  a  s  a  «  « 

ft    ^       ?n    O  O  O  O  O  O  O  O  O  O  O 

2H|S  B  S  B  B  B  B  B  B  B  B 

h      2       r?     O)  CD  CD  CD  CD  CD  CD  CD  CD  CD  CD 

M£^  ^  A  A  A  A  J  A  HH  A  A 


Eh       <o    m 

OtfO  O  O  O  O  O  O  O  O  O  O 


^^PhPhPhPhPhPhPhPhPhPhPh 

Eh     W 

p^flclclplcicjflflplclfl 

ajcrj.-^o  o  o  o  o  o  o  o  o  o  o 

w    r..     ?   =  fl  S  P.  fl  S  P  S  Fl  S  S 


OfkJcD  CD  CD  CD  CD  CD  CD  CD 


Z     Eh  f3 

O  oj  P 

el         03       5H  W  H-H  ^     vn 


S  *«S    «s    .".S    »2    (is  J.S        .5        S    --  ">  a    -§  - 

9  Ssa  §|  §|  S|  §  s  ||  «s  §§  a  £  §^  «  - 

5  3h_jHJHJ^J                    rn  c3  OS  C?H  fl 

P  gCD                  CD                 CD                  CD.S  fl  CD  <B^_,  CD 


0 

-1 

CD 

CD 

CS 

fl 

fl 

•  rH 

— 

at 

oS 

— 

S 

fl 

a 

COTTONY    ROT   OF   LEMONS   IN    CALIFORNIA 


251 


On  Citrus  Twigs. — These  are  very  readily  infected  artificially  with 
the  cottony  rot  fungus.  Infection  has  been  repeatedly  caused  by  using 
apothecia  and  spores,  mycelium  and  sclerotia.  In  the  inoculation  of  the 
twigs  a  sloping  cut  through  the  bark  into  the  wood  was  made  and  con- 
siderable of  the  mycelium  was  placed  in  this  injury.  To  insure  the  best 
infection,  the  injury  was  then  wrapped  with  paraffin  waxed  paper  to 
prevent  drying  out.  By  this  method  the  injured  part  of  the  wood 
and  bark  would  be  rather  easily  attacked  by  the  fungus,  starting  as  a 
saprophyte.     From  the  injury  the  infection  gradually  spreads  until 


Fig.  9. — Cottony  rot  affecting  the  blossoms  of  a  Eureka  lemon  tree.     This 
condition  only  appears  under  certain  very  favorable  conditions. 

it  completely  encircles  the  branch.  Gumming  takes  place,  especially 
toward  the  margin  of  the  infection.  The  affected  bark  of  the  citrus 
twig  differs  from  normal  tissue  in  color,  being  greyish  or  cinerous. 
Sclerotia  may  or  may  not  be  produced  in  the  infected  bark  (Fig.  7b), 
which  is  at  first  soft,  becoming  at  length  fibrous.  The  infection  may 
extend  1  or  2  inches  in  length  of  twig,  gradually  surrounding  and 
killing  it.    The  infection  appears  to  die  out  when  healing  takes  place. 

The  conclusion  drawn  from  these  experimental  inoculations  with 
different  strains  of  Sclerotinia  is:  (1)  That  the  different  strains  used 
must  belong  to  a  common  species,  or  (2)  that  the  citrus  host  must 
be  especially  susceptible  to  infection  from  different  species  of  this 
genus. 


252 


UNIVERSITY   OF    CALIFORNIA — EXPERIMENT    STATION 


METHODS  OF  CONTROL 

In  Field. — The  spore  stage  of  the  fungus  occurs  in  the  citrus  groves 
(Fig.  11),  never  in  the  packing  house.  Certain  favorable  conditions 
are  necessary  in  the  grove  for  the  fungus  to  continue  to  perpetuate 
itself.  Vegetable  organic  matter  must  be  present  on  which  the  myce- 
lium can  develop  during  the  rainy  season.  This  it  does  as  a  sapro- 
phyte on  the  dead  parts  of  plants,  gradually  making  its  way  into  the 
tissue,  killed  probably  by  the  secreted  enzymes  and  acids.     In   this 


Fig.  10. — The  vegetative  stage  of  cottony  rot  as  it  appears  growing 
on  a  vetch  cover  crop. 


vegetative  stage  the  sclerotia  which  later  produce  the  spore  stage,  are 
formed.  Without  this  organic  matter  in  the  orchard  in  a  few  years 
the  spore  stage  would  cease  to  be  formed.  What  has  just  been  said 
should  not  in  any  way  be  regarded  as  a  recommendation  to  cease  grow- 
ing cover  crops.  Most  growers  would  prefer  to  lose  some  lemons  by 
cottony  rot,  even  though  indirectly  brought  about  through  the  con- 
tinual growing  of  cover  crops,  rather  than  to  sacrifice  the  health  and 
vigor  of  the  orchard  by  not  adding  vegetable  organic  matter  to  the 
soil  by  green-manure  crops.  Observations  on  fruit  grown  in  clean 
cultivated  groves  show  little  or  no  cottony  rot.     The  groves  where 


COTTONY    ROT   OF   LEMONS   IN    CALIFORNIA  253 

the  decay  is  worse  are  those  that  may  have  the  best  care  and  in  which 
a  cover  crop  or  the  natural  vegetation  is  allowed  to  grow.  A  cover 
crop  gives  almost  ideal  conditions  for  the  sclerotia  and  the  spore  stage 
to  develop. 

Spraying  for  this  trouble  is  not  to  be  recommended.  All  observa- 
tions thus  far  show  no  fruit  to  be  infected  on  the  trees,  and  the  fruit 
can  be  more  economically  disinfected  in  the  wash  water  at  the  packing 
house.  Many  spores  of  the  fungus  must  lodge  on  the  surface  of  fruit 
and  some  must  germinate  while  the  fruit  is  attached  to  the  tree,  but 
from  the  experiments  already  considered,  probably  no  infection  could 
take  place  unless  it  might  be  at  the  blossom  and  button  of  stem.  This 
is  unlikely,  as  orchard  conditions  would  be  much  less  favorable  than 
those  of  the  packing  house  or  moist  chamber.  Still  it  may  not  be  im- 
possible for  a  slight  infection  to  occur  at  the  places  just  mentioned. 
This  may  account  in  part  for  the  difficulty  of  disinfection  in  the 
wash  water. 

The  disease  on  the  branches  of  citrus  trees,  while  of  scientific  in- 
terest, is  under  ordinary  conditions  of  no  great  economic  importance. 
Where  present  the  diseased  twigs  can  be  readily  cut  out  when  the 
lemon  trees  are  pruned. 

In  Packing  House. — The  use  of  germicides  in  the  wash  water  as 
a  control  for  the  brown  rot,  Pythiacystis  citropkthora,  is  a  general 
practice.  The  chemical  used  most  is  copper  sulphate,  which  is  very 
efficient  as  a  check  for  brown  rot,  but  is  not  equally  effective  against 
the  spores  of  the  blue  and  green  mold,  Penicillum  italicum  and  P. 
digitatum,  or  those  of  cottony  rot,  Sclerotinia.  The  chief  reason  for 
this  is  that  the  spores  are  given  off  in  dry  masses  containing  air  which 
forms  a  film  about  many  of  the  spores,  preventing  their  being  wet  by 
a  watery  solution  of  the  germicide.  This  is  well  illustrated  by  the 
following  experiment  to  test  the  action  of  copper  sulphate  as  a 
germicide : 

April  5th,  1912.  A  single  apothecium  was  placed  without  crushing 
it  in  10  cc.  of  sterilized  water  contained  in  a  test  tube.  After  15  min- 
utes the  water  was  well  infected  with  cottony  rot  spores,  and  the 
apothecium  was  removed.  From  this  spore-laden  liquid,  one  loopful 
(2  mm.  diameter  loop)  was  placed  in  each  of  the  test  tubes  containing 
10  cc.  of  copper  sulphate  to  be  tested.  After  the  required  length  of 
time  that  the  copper  sulphate  solution  was  to  act  on  the  spores,  the 
entire  contents  of  the  tube  was  poured  into  a  flask  of  bread  and  prune 
juice  medium.  The  following  shows  the  strengths  in  per  cent  used 
and  the  time  that  the  spores  were  acted  upon  by  the  germicide : 


254 


UNIVERSITY   OF    CALIFORNIA EXPERIMENT    STATION 


Per  cent 

3  min. 

5  min. 

10  min. 

15  min. 

1/5 

5  tubes 

5  tubes 

5  tubes 

5  tubes 

1/10 

5  tubes 

5  tubes 

5  tubes 

5  tubes 

1/25 

5  tubes 

5  tubes 

5  tubes 

5  tubes 

1/50 

5  tubes 

5  tubes 

5  tubes 

5  tubes 

April  11th,  1912.    All  the  different  strengths  tested  in  bread-prime 
juice  medium  showed  characteristic  growth  of  cottony  rot.     Hence 


ili* 


Fig.  11. — Peziza  or  spore  stage  of  cottony  rot  fungus  growing  under  a 
vetch  cover  crop  in  orchard  soil. 

even  the  longest  time,  15  minutes,  and  the  greatest  strength,  y5  per 
cent,  wrere  not  sufficient  to  kill  all  the  spores  treated.  It  is  not  strange, 
therefore,  that  under  the  usual  commercial  conditions,  spores  of  the 
fungus  should  occasionally  pass  through  the  disinfection,  with  copper 
sulphate,  without  being  killed. 

The  chief  difficulty  in  using  disinfectants  is  to  find  one  that  will 
kill  the  spores  without  injuring  the  lemons.  Formaldehyde  is  much 
more  efficient  as  a  germicide  for  cottony  rot  than  copper  sulphate,  but 


COTTONY    ROT    OF    LEMONS    IN    CALIFORNIA 


255 


its  use  is  not  to  be  encouraged  because  of  its  injury  to  the  fruit,  fol- 
lowed by  infection  with  the  two  species  of  Penicillium,  the  green  and 
blue  mold. 

In  some  of  the  preliminary  work  it  was  shown  that  by  using  a 
wash  of  alcohol  preceding  that  of  the  copper  sulphate,  more  satis- 
factory results  could  be  secured  in  killing  the  spores  of  the  cottony 
rot.  This  method  was  tested  out  in  a  limited  way  at  one  of  the  pack- 
ing houses.  The  results  are  given  in  another  paragraph  and  suggest 
that  further  experimentation  may  prove  that  alcohol  has  some  bene- 
ficial results  in  separating  the  spores  from  each  other  so  that  they  may 
be  the  more  readily  acted  upon  afterwards  by  the  germicide. 

The  experiment  was  divided  into  three  parts:  (1)  Fruit  was 
washed  in  tap  water;  (2)  fruit  was  washed  in  tap  water  acidified  with 
255  cc.  (about  y2  pint)  of  commercial  sulphuric  acid  to  1000  gallons 
of  water,  afterwards  copper  sulphate  was  added  to  make  it  1/50  per 
cent,  which  is  as  strong  a  solution  of  bluestone  as  can  be  used  without 
danger  of  injuring  the  fruit;  (3)  fruit  was  soaked  in  25  per  cent 
denatured  alcohol  for  five  minutes  and  then  washed  as  indicated 
under  "2." 

The  object  of  adding  the  sulphuric  acid  was  to  neutralize  partly 
the  alkalinity  in  the  wash  water.  This  will  be  considered  in  detail  later 
on  in  this  paper.  The  experiment  extends  over  a  portion  of  two 
seasons  and  is  best  summarized  as  follows : 


Date 

Treatment 
No.  1* 

Treatment 
No.  2f 

Treatment 
No.  3} 

Results 
40-60  days 

3/7/14 

12  boxes 

12  boxes 

12  boxes 

No  cottony  rot. 

3/17/14 

12  boxes 

12  boxes 

12  boxes 

1  box  cottony  rot  in  Treat 
ment  2. 

3/24/14 

10  boxes 

10  boxes 

10  boxes 

No  cottony  rot. 

4/4/14 

10  boxes 

10  boxes 

10  boxes 

No  cottony  rot. 

4/ J  5/14 

10  boxes 

10  boxes 

10  boxes 

No  cottony  rot. 

12/3/14 

8  boxes 

8  boxes 

8  boxes 

1  box  cottony  rot  in  Treat 
ment  1. 

1/7/15 

8  boxes 

8  boxes 

8  boxes 

No  cottony  rot. 

1/14/15 

8  boxes 

8  boxes 

8  boxes 

No  cottony  rot. 

*  Column  1.     Contains  the  number  of  field  boxes  washed  in  plain  tap  water. 

t  Column  2.  Contains  the  number  of  field  boxes  washed  in  acidified  water, 
copper  sulphate  solution  1/50  per  cent. 

X  Column  3.  Contains  the  number  of  field  boxes  treated  first  with  25  per 
cent  denatured  alcohol,  then  washed  in  acidified  copper  sulphate  solution  1/50 
per  cent. 


The  work  so  far  is  not  at  all  conclusive.  No  cottony  rot  developed 
where  alcohol  was  used,  while  it  did  appear  in  a  few  cases  where  the 
copper  sulphate  was  used  in  the  water  or  where  only  water  was  used. 


256  UNIVERSITY   OF    CALIFORNIA — EXPERIMENT    STATION 

This  may  have  been   accidental.     The   experiment   was   inconclusive 
because  so  little  cottony  rot  appeared  to  be  present  in  the  fruit. 

Germicidal  Efficiency  of  Wash  Waters. — Copper  sulphate  1/50  per 
cent  is  the  best  germicide  thus  far  tested  for  general  use  in  the  dis- 
infection of  lemons,  although  it  is  not  equally  effective  in  the  killing 
of  the  spores  of  the  different  rot  producing  fungi,  even  when  kept  at 
the  above  strength.  The  fruit  goes  through  the  washer  in  a  rather 
short  length  of  time,  so  that  it  is  very  essential  that  the  strength  of 
the  copper  sulphate  be  kept  uniformly  as  strong  as  can  safely  be  used. 
The  following  paragraphs  were  written  on  the  preparation  of  dis- 
infecting wash  water  by  Mr.  E.  E.  Thomas,  of  the  Citrus  Experiment 
Station,  who  has  been  giving  considerable  study  to  the  chemistry,  and 
especially  to  the  alkalinity,  of  the  wash  waters  used  in  many  of  the 
lemon  packing-houses  in  California : 

1 '  The  main  trouble  in  the  use  of  copper  sulphate  in  the  wash  water 
as  a  disinfectant  has  been  in  keeping  the  strength  constant  during 
the  day.  In  the  morning  it  may  be  one  strength,  at  noon  another,  and 
at  night  still  another.  This  should  not  be  the  case,  but  the  percentage 
of  copper  sulphate  in  the  water  should  remain  uniformly  the  same 
throughout  the  day  in  order  to  give  a  thorough  and  satisfactory 
disinfection  of  the  fruit. 

"Most  of  the  wash  waters  used  for  washing  lemons  in  the  packing- 
houses of  California  contain  carbonates  in  solution  that  give  the  water 
an  alkaline  reaction.  These  carbonates  precipitate  a  part  of  the 
copper  sulphate  as  basic  copper  carbonate  and  thus  reduce  the  copper 
sulphate  in  solution  and  lower  the  germicidal  strength  of  the  solution. 

"This  factor  can  be  largely  overcome  by  the  addition  of  an  acid, 
such  as  sulphuric  acid,  to  the  water  before  the  copper  sulphate  has 
been  poured  into  it  in  the  wash  tank.  The  addition  of  this  acid  can 
be  accomplished  without  injury  to  either  fruit  or  tank,  as  the  desired 
result  can  be  obtained  by  neutralizing  only  two-thirds  or  three-quarters 
of  the  alkali  in  the  water.  It  would  be  advisable  to  add  from  eight 
to  ten  fluid  ounces  of  91.5  per  cent  sulphuric  acid  to  a  tank  containing 
one  thousand  gallons  of  water  which  has  two  hundred  parts  calcium 
carbonate  per  million.  Upon  adding  copper  sulphate  to  water  treated 
in  this  manner  very  little  basic  copper  carbonate  is  formed. 

"The  other  factor  which  lowers  the  percentage  of  copper  sulphate  in 
solution  is  the  addition  of  hydrant  water  which  it  seems  necessary  to 
add  from  time  to  time  during  the  day  to  take  the  place  of  that  removed 
by  the  lemons.  The  addition  of  this  water  not  only  lowers  the  per- 
centage of  copper  sulphate  in  solution,  but  also  adds  more  alkali,  thus 


COTTONY   ROT    OF    LEMONS  IN    CALIFORNIA 


257 


causing  more  basic  copper  carbonate  to  be  formed,  which  still  further 
weakens  the  solution. 

"In  the  study  of  this  question  it  seemed  necessary  to  find  some 
method  by  which  the  water  that  is  added  during  the  day  can  be  made 
to  contain  the  proper  amount  of  copper  sulphate.  At  present,  the  best 
way  that  can  be  suggested  to  accomplish  this  end  is  the  installation  of 
a  second  tank,  from  which  the  water  needed  during  the  day  can  be 
drawn.  This  tank  should  be  filled  each  morning  with  a  solution  of 
copper  sulphate  of  the  same  strength  as  that  used  in  washing  the 
lemons  and  the  solution  should  be  made  in  the  same  manner. ' ' 

Other  Suggestions. —  (a)  Keep  the  humidity  of  the  tents  as  low  as 
possible  without  injury  to  the  fruit,  (b)  Systematic  inspection  of  stored 
fruit,  especially  from  those  groves  that  in  the  past  have  shown  cottony 
rot.  (c)  Do  not  use  packing  boxes  that  have  in  the  past  been  infected 
with  cottony  rot  unless  these  are  thoroughly  disinfected  or  the  stained, 
infected  portions  are  replaced  with  new  wood.  From  some  preliminary 
work  it  has  been  shown  that  five  per  cent  solution  of  formalin  is  the 
best  disinfectant  to  use  in  sterilizing  boxes.  The  following  experiment 
shows  the  use  of  different  germicides  in  the  treatment  of  portions  of 
boxes  that  became  infected  with  cottony  rot.  The  pieces  measured 
about  three  inches  long  by  one  inch  wide.  There  were  five  pieces  tested 
in  each  case.  The  results  given  were  after  the  treated  pieces  had  been 
in  moist  chamber  two  weeks. 


Germicide 
Mercuric  chloride  1/10% 
Mercuric  chloride  1/10% 
Mercuric  chloride  t/1^% 
Formalin  5% 
Formalin  5% 
Formalin  5% 
Check 

Boiling  water 
Boiling  water 
Boiling  water 
Copper  sulphate  5% 
Copper  sulphate  5% 
Copper  sulphate  5% 

*  Copper  sulphate  was 


Time 

10  seconds 
1  minute 
5  minutes 

10  seconds 
1  minute 
5  minutes 
Untreated 

10  seconds 
1  minute 
5  minutes 

10  seconds 
5  minutes* 
5  minutes 


Results 
No  growth. 
No  growth. 
No  growth. 
No  growth. 
No  growth. 
No  growth. 
Cottony  rot. 

Growth  of  penicillium  and  no  evi- 
dence of  cottony  rot. 

Growth    of    penicillium    and    evi- 
dence of  cottony  rot. 


rinsed  off  at  end  of  treatment  five  minutes. 


(d)  Contact  fruit  should  be  kept  separate  from  other  fruit  where 
frequent  attention  can  be  given  it.  Some  houses  have  found  it  desir- 
able to  employ  specially  trained  men  to  handle  virulent  decays  like 
cottony  rot  and  brown  rot. 


258  UNIVERSITY   OF    CALIFORNIA— EXPERIMENT    STATION 


GENERAL  CONCLUSIONS 

Research  upon  this  disease  has  established  the  following  facts : 

1.  Repeated  inoculations  have  proved  that  the  eottony  rot  is  caused 
by  a  species  of  Sclerotinia,  probably  Sclerotinia  libertiana. 

2.  That  this  fungus  not  only  attacks  the  lemon  fruit  but  also  the 
twigs  of  mature  and  small  orchard  trees,  nursery  stock,  and  sweet 
and  sour  orange  seed  bed  stock. 

3.  The  white  cottony  filaments  (mycelium)  or  the  vegetative  part 
of  the  fungus,  are  able  to  enter  and  destroy  a  perfectly  sound  lemon 
at  any  point  of  contact,  no  abrasion  being  necessary. 

4.  Several  strains  of  Sclerotinia  libertiana  from  different  localities 
of  the  United  States,  when  inoculated  in  the  fruit  and  twigs  of  lemons 
are  pathogenic,  showing  infection  similar  in  all  characteristics  to  that 
of  lemon  cottony  rot  isolated  from  citrus  fruits  and  twigs. 

5.  Attempts  to  inoculate  the  healthy  uninjured  skin  of  a  lemon 
with  spores  applied  with  an  atomizer  over  the  entire  fruit,  have  failed 
except  at  the  stem  and  blossom  ends,  and  rarely  at  points  where  two 
fruits  are  in  contact. 

6.  The  fungus  has  repeatedly  been  carried  through  its  entire  lite 
history  from  ascospore  to  ascospore.     No  other  spore  stage  has  been 

observed. 

7.  Strains  of  Sclerotinia  isolated  from  bean,  cucumber,  lettuce, 
vetch  wild  lettuce  (Latuca  scariola),  citrus  twigs,  avocado  twigs. 
tomato,  and  eggplant,  appear  to  be  identical  with  that  causing  the 
decay  of  lemon  fruits  known  as  the  cottony  rot. 

8  No  more  satisfactory  control  has  been  discovered  than  disin- 
fecting the  fruits  by  the  use  of  a  wash  water  containing  1/50  per  cent 
solution  of  copper  sulphate,  a  method  in  general  use  for  the  control 


of  brown  rot. 

9  It  has  been  found  important  to  neutralize  the  wash  water  by 
use  of  sulphuric  acid  before  the  copper  sulphate  is  added,  this  being 
necessary  to  prevent  the  breaking  down  of  the  copper  sulphate  and 
the  weakening  of  the  disinfecting  solution. 

10  In  replenishing  the  solution  in  the  wash  tank,  more  of  the  dis- 
infecting solution  properly  made  and  of  the  required  strength  should 

be  added.  .    .  ,  . 

11    Other   important  factors  in   controllmg  the  injury   from   tins 
disease  in  packing  houses  are:    (1)   the  sterilization  of  storage  boxes, 

(2)  frequent  inspection,  especially  of  stored  fruit  from  infested  groves. 

(3)  the  isolation  of  fruit  that  may  have  been  infected  by  contact. 


